Electrode structure for electrical space discharge tubes



Jan. 25; v1949. R. wooD- 2,459,861

ELECTRODE STRUCTURE FOR ELECTRICAL SPACE DISCHARGE TUBES Filed March 21 1946 1 FIGJ //v VENTOI? Poss W000 Patented Jan. 25, 1949 ELECTRODE STRUCTURE FOR. ELECTRICAL SPACE DISCHARGE TUBES Ross Wood, Waltham, Mass, assignor to Raytheon Manufacturing Company, Newton, Mass, a corporation of Delaware Application March 21, 1946, Serial No. 655,967

3 Claims. 1 This invention relates to space discharge tubes, and particularly to tubes of small dimensions suitable for use in portable devices, such as hearing aids, pocket radios, and similar constructions.

In conventional tubes of the type to which t e invention relates, the plates are usually fiat mem bers separated from the screen grid by a suppressor grid. The function of the suppressor grid is to prevent the interchange of secondary emis sion between the plate and the screen grid. In small tubes the accomplishment of this function is limited by the very small spacings involved. In large tubes high plate resistance and increased voltage amplification is obtainable by increasing the distance from cathode to plate, but such increase in the spacing would defeat the objective of small tube size.

A further difficulty arising in construction of tubes of small diameter resides in the fact that conventional constructions are not suitable for use at radio frequencies without additional shielding. There is insufficient room to provide such shielding within the bulb; hence, external shieldmaterial, containing an electrode assembly which comprises a filamentary cathode 5, a pair of anodes 6 and I, and a plurality of grids, herein 8, 8 and Ill. The anodes 6 and l are in the form of rods of relatively small diameter, for example, 9.015 inch. The two anodes are electrically connected together at either end thereof. The control grid 8 and the screen grid 9 surround the filamentary cathode 5 intervening between said cathode and the anodes 6 and 1. The suppressor grid It is in the form of an imperforate member of sheet metal surrounding the grids 8 and 9 and having surfaces defining a. pair of recesses,

' one for each of said anodes. The suppressor grid cathode.

mg is required. Such external shielding is not satisfactory because of the high value of elec trostatic capacitance between the grid and plate.

It is among the objects of the present invention to overcome the difliculties mentioned in the foregoing and provide a tube of relatively small diameter, having high plate resistance, and in which the electrostatic capacitance between the control grid and the plate is substantially reduced.

It is a further object of the present invention to provide for the complete shielding of the electrode elements within the tube Without increasing the overall dimensions thereof.

The above and other objects and features of the invention will be made fully apparent to those skilled in the art from a consideration of the following detailed description taken in conjunction with the accompanying drawing in which:

Fig. 1 shows in side elevation, with parts broken away to show internal construction, a tube incorporating one illustrative embodiment of the invention;

Fig. 2 is a detail showing in perspective a portion of the electrode assembly of the tube illustrated in Fig. 1;

Fig. 3 shows on a greatly enlarged scale a section taken on the line 3-3 of Fig. 1; and

Fig. 4 shows, on an enlarged scale, a section taken on the line 4-4 of Fig. 1.

Referring to the drawing, the illustrative embodiment of the invention shown comprises an evacuated envelope 4 of glass or other suitable The filamentary cathode, the anode rods and grids are suitably spaced by upper and lower insulating spacing members H and 12 of mica or other suitable insulating material, provided with suitably spaced holes through which the filamentary cathode, the anode rods, and the supporting rods for the grids extend. At their edges, the spacing members H and ii! are provided with resilient projections l3 engaging the wall of the envelope 4. The ends of the rods project through the insulating members I2 and II and these -members are rigidly secured together by tabs welded to said projecting ends of the rods, the insulating members being held apart the proper distance bythe suppressor grid 10.

One of the supportingrods !5 for the suppressor grid I0 is connected at its lower end to a lead-in conductor l6, and its upper end is connected to the filamentary cathode 5 through a resilient metal rod H, which rod supports the upper end of the filament under suitable tension. The lower end of the filament is connected to the cross bar is of a supporting yoke having upwardly extending arms 19. The upper ends of the arms l9 extend through and are rigidly secured in the lower insulating member l2 at points outside of the suppressor grid W. The cross bar I8 is welded to and supported by a lead-in conductor 20. A pair of shields 2|, each having one end welded to one of the arms l9, extend between the projecting ends of the anodes 6 and l and the adjacent projecting ends of the grid support rods. In Fig. 1 the foremost of these shields has been removed for the sake of clearness. One of the anode rods, herein 6 is connected by a conductive strip 22 to a lead-in conductor 23, which conductor is bent outwardy at its upper end to provide a point of attachment on the outside of and spaced from the adjacent shield 2!. The other anode 1 is connected at its upper end to the anode rod 6. For convenience the two anode rods 6 and I may be the two arms of a single U-shaped member, the bent portion of the U providing the electrical connection between the two rods. One of the supporting rods 24 for the control grid 8 is connected at its lower projecting end to a lead-in conductor 25. Similarly the lower projecting end of one of the supporting rods 26 of the screen grid 9 is connected to a lead-in conductor 21. A getter assembly 28 may be connected to the upper projectin end of one of the supporting rods Him the suppressor grid, and a shield 29'may also'be welded to said projecting end of said rod, which shield extends for a sufficient distance to intervene between the upper projecting ends Of the grid supporting rods and the upper projecting portion of the anodes 6 and l.

The electrode assembly described in the foregoing is inserted as a unit within a glass tube having walls which are straight taken ina longitudinal direction, and of ellipsoidal form taken in transverse section. The lower end of this tube is heated and softened sufficiently so that it may be pressed to make the fiat external press or seal 39 hermetically sealing all of the external connections provided by the lead-in conductors I6, 20, 23, 25 and 21.

The construction described in the foregoing provides a tube in which the anode plates are in the form of rods of relatively small-diameter, which construction reduces the electrostatic capacitance between the control grid and. plate and at the same time increases the plate resistance.

The suppressor grid provides a shield member having surfaces defining recesses which extend around'each of the anode rods, surfaces providing a shield. This shield is connected to the cathode and provides a potential minimum between the screen grid and the plate, This is due not only to the electrostatic effect of the shield, but also due to the fact that the shield confines the electron stream to form a. beam, thus further reducing the potential due to space charge. The shield member also reduces the electrostatic capacitance between the control grid and the plate. Furthermore, the suppressor grid effectively shields theentire structure from external coupling efiects. A significant part of the gridplate capacitance in small tubes results from the electrostatic path from the grid-lead in the'press along the cross-section of the tube walls and through space to the plate. by the present structure isolates the plate from The shield provided 4 the bulb and thus eliminates this contribution to grid-plate capacitance.

While there has been herein described a preferred embodiment of the invention, other embodiments within the scope of the appended claims will be apparent to those skilled in the art. from ,a fconsiderationrof the *embodiment shownand the teachings hereof.

What is claimed is:

1. An electrical space discharge tube comprising a cathode, apairof anodes disposed on opposite'sides of said cathode, a pair of grids surrounding said cathode and intervening between said 'cathode and anodes, and a suppressor grid comprising an imperforate sheet of metal surrounding said grids and having surfaces defining a pair of recesses, one for each of said anodes, said surfaces providing shields extending around the sides-of said anodes remote from said cathode.

2. An electrical space discharge tube comprising a filamentary cathode, a pair of anode rodsof relatively small diameter disposed on opposite sides of said'cathode, a pair of rids surrounding said cathode and intervening between said cathode and anode rods, and a suppressor grid comprising an. imperforate sheet of metal -'surrounding said grldsand havings'urface's defining apair of recesse -one for each of said'anode rods, said surfaces providing shieldsexten'din'g around the sides of said anode rods remote from said cathode.

3. An electrical space discharge tube compris-' ing a filamentary cathode, apai'r of anode rods disposed on opposite sides of'and extending parallel with saidcathode, a pair 'of' grids surrounding said cathode and intervening between said cathode and anode rods, and a su'ppres'so'r grid comprising an imperforate sheet of metal surrounding said grids and have surfaces "defining a pair of recesses, one for each of saida'node's, said surfaces providing shields extending around the sides of said anode rods remote from said cathode. v

ROSS WOOD.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,030,931 Parker Feb. :18, 1936 2,061,254 Rockwood, Jr. 'Nov. 17, 1936 2,146,016 Herold Feb. 7, 193 9 2,250,184 Miller July 22, 19 1 1 

